Conventionally, there has been known a particulate detection system which detects the amount of particulates (e.g., soot) contained in exhaust gas discharged from an internal combustion engine such as a diesel engine (see Japanese Unexamined Publications No. 2012-220423 and No. 2012-194078). In such a particulate detection system, clean air is externally supplied to a sensor attached to an exhaust pipe, discharge is generated in the clean air so as to produce ions, and the ions are jetted together with the clean air to thereby produce a gas flow from the interior of the sensor toward the outside of the sensor. The ions are caused to adhere to particulates contained in exhaust gas (gas under measurement) introduced into the sensor by the gas flow so as to produce electrified particulates. The electrified particulates are caused to flow out to the outside of the sensor (into the interior of the exhaust pipe), and the ions having failed to adhere to the particulates are collected. On the basis of the magnitude of a sensor current corresponding to the amount of flowed-out charge; i.e., the amount of charge carried by the ions which have adhered to particulates and flowed out to the outside of the sensor (into the interior of the exhaust pipe), the amount of particulates contained in the exhaust gas is determined. Examples indexes of the amount of particulates contained in the exhaust gas include the weight of particulates contained per unit volume of the exhaust gas (mg/m3, hereinafter also referred to as the “volumetric particulate weight”) and the number of particulates contained per unit volume of the exhaust gas (pcs/m3, hereinafter also referred to as the “volumetric particulate number). In the following description, the “volumetric particulate weight” and the “volumetric particulate number” will be collectively referred to as the “volumetric particulate amount.” Since the amount of ions which adhere to particulates correlates with the amount of particulates contained in the exhaust gas, the particulate detection system can determine the amount of particulates contained in the exhaust gas from the sensor output current value corresponding to the amount of ions having flowed out.
Also, there has been known a particulates sensor configured to produce a flow of a gas under measurement flowing through the interior of a sensor, without using externally supplied clean air, by utilizing the flow of exhaust gas (external gas) which flows outside the sensor, and to generate discharge within the gas under measurement, thereby producing ions (see Japanese Unexamined Publication No. 2015-129711, FIG. 6, etc.). The particulate sensor of this type operates in the same manner as the above-described particulate detection system. Specifically, ions are caused to adhere to particulates contained in exhaust gas (gas under measurement) so as to produce electrified particulates. The electrified particulates are caused to flow out to the outside of the sensor, and the ions having failed to adhere to the particulates are collected. The amount of particulates contained in the exhaust gas is determined on the basis of the magnitude of the sensor current corresponding to the amount of flowed-out charge; i.e., the amount of charge carried by the ions which have adhered to particulates and flowed out to the outside of the sensor.
Also, WO 2012/161754 discloses a known particulate detection system of a different scheme in which ions are not generated, and the amount of particulates is determined through use of electrified particulates in exhaust gas.
The particulate detection systems of these schemes can detect in real time the mount of particulates contained in exhaust gas; for example, the volumetric particulate weight (unit: mg/m3), which is the weight of particulates contained per unit volume of the exhaust gas.